Cyanuric acid can be prepared by heating urea, biuret or mixtures thereof in the presence of a solvent. In the literature, a great number of different solvents, sometimes referred to as distributing agents, are described, many of which are polar and, water-soluble to varying degree. Examples of suitable distributing agents are sulphones, like dimethylsulphone, dipropylsulphone and sulpholane, chlorocresols, N-methylpyrrolidone, 5-methyl-2-oxazolidinone and methyl-substituted cyclohexanols, as well as the reaction products of these solvents with urea and biuret. Since cyanuric acid is only slightly soluble in those distributing agents, a suspension of the product cyanuric acid in the distributing agent is obtained when urea, biuret or mixtures thereof are heated in those distributing agents. The cyanuric acid can be separated therefrom.
It has been discovered that cyanuric acid prepared and isolated from the aforementioned reaction mixtures shows traces of the distributing agent used enclosed in the solid product, which are removed only with great difficulty. It is not possible, for instance, to remove these residues of the distributing agent by washing them out, unless such hugh amounts of washing liquid are used as to cause a large portion of the cyanuric acid to become dissolved, which, of course, is highly uneconomical. Yet, it is desirable for many applications that the cyanuric acid be freed from these residues of the distributing agent.
The invention relates to a process for purifying cyanuric acid which contains residues of a polar distributing agent enclosed in the solid cyanuric acid.